The invention relates generally to rolls for treating webs of material and, more particularly, to apparatus for controlling the temperature of a heated roll as it is heated-up to, or cooled-down from, its operating temperature.
For some time, heatable rolls have been provided with internal canals through which a fluid heat carrier medium is conducted. The inner surfaces of these canals transfer heat from the fluid medium to the roll from the inside. In many cases, these canals are formed by deep holes extending parallel to the longitudinal axis of the rolls. The holes may have cross connections at their ends to provide a meandering flow path for the fluid medium, which ensures a uniform temperature distribution. Peripherally-drilled rolls, which are employed in paper-making machines and calendars, can now be made with working widths up to 7,500 mm. More frequently, however, for large working roll widths, hydraulically supported rolls are employed in which a hollow roll is rotatable about a stationary crosshead. In this type of roll, the inside roll surface to which heat is transferred is the inner circumference of the hollow roll, and therefore, manufacturing problems that would arise in creating the inside roll surface, comparable to those in rolls having deep-drilled holes, are obviated. The heat is transmitted through the heat carrier medium, which is located in the interior clearance space between the hollow roll and crosshead. This medium is in contact with the inner circumference of the hollow roll to transfer heat to the roll. These hydraulically supported rolls can be manufactured with the largest dimensions that can be employed in practice, i.e., up to about 10 m in length and 1 m in diameter.
Such large roll bodies are formed by casting. In the casting process, a structure develops that has certain restrictions with respect to further temperature stress. While the problem of the thermal stress fundamentally exists in all heated rolls, it is particularly pronounced in cast hollow rolls of the gray iron or chilled cast type. These materials are brittle and their structure has a tendency to fracture if subjected to tensile stress; the fracture points can be starting points for larger cracks. Especially dangerous in this regard are chilled cast tubes or cylinders because of the pattern of internal stress formed in their manufacture. The producers of such rolls prescribe a maximum heating-up rate of about 2.degree. C./min when heating from one side. Otherwise, a stress may result that exceeds the strength of the structure due to the thermal stress generated by a larger temperature differential being superimposed on the internal stress.
Heating-p rates on the order of magnitude mentioned above require waiting two to three hours until the rolls are heated to an operating surface temperatures of 200.degree. to 300.degree. C. before production can start. Since rolls of the type under discussion are usually parts of larger systems, corresponding shutdown periods of the larger systems, with the attendant loss in economic efficiency, ensue.
In addition to the heating-up process, the abovementioned problems also occur when rolls are being cooled-down. Fast cool-down may be necessary so that the roll can continue to operate at low temperature in the event of a product change or, upon a change of rolls, so that the temperature can be lowered to a value that permits disassembly of the roll.